1. Field of the Invention
A device for determining position coordinates on a flat surface.
2. Description of the Prior Art
Many existing devices for determining position coordinates are constructed with a tablet, a stylus pen and the like. One group of such devices includes a tablet which has a plurality of conductive lines located at equal intervals, and a stylus pen with electromagnatic induction windings to which signals are applied. A conductive line facing the stylus pen picks up a signal induced from the stylus pen, whereby the position coordinates of the pen relative to the plurality of conductive lines are determined.
Another group of known devices is based on the principle of electromagnetic induction. The position coordinates of a cursor having an excitation coil are determined by comparing the phase of a signal induced in a loop-shaped sensing line on the tablet with the phase of a signal applied to the excitation coil of the cursor to determine whether it is placed within or outside of the line.
In these types of devices using electromagnetic induction, the circuit for detecting position coordinates can be made comparatively simply because the interval between scanning or sensing lines is equal to the resolution of the position coordinates. But on the other hand, the devices are easily interfered with by external noise because the devices employ a high impedance sensor.
It is another disadvantage to make the scanning circuit comprising a large number of lines used within and it is difficult to manufacture a large sized tablet for the purpose of increasing the resolution.
The devices which determine the coordinates by continuously detecting the phase difference between excitation signal and induced signal also have the disadvantage that the absolute coordinate position can not be obtained relative to the tablet because these values are determined by signals which exhibit a repetitive 180.degree. phase shift. Once the cursor is moved away from the tablet, the established coordinate system is never obtained again.
These characteristics can be prevented if the position coordinates are manually put into a memory when the cursor is removed, or if the sensing line for detecting the absolute coordinates position are newly installed.
But it is disadvantageous that operation require rather difficult procedures or the tablet be made complicated in structure.
In these existing coordinate determining devices, the signal of the sensor and the applied signal should be synchronized. Also it is required that wire made of a plurality of lines which is connected to the signal generator should be provided for the stylus pen or cursor.